The present invention relates generally to x-ray tubes and, more particularly, to a cathode configuration therefor.
Presently available medical x-ray tubes typically include a cathode assembly having an emitter and a cup. The cathode assembly is oriented to face an x-ray tube anode, or target, which is typically a planar metal or composite structure. The space between the cathode and anode is evacuated.
A disadvantage of typical cathode designs is that the emitter, which typically comprises a helically coiled tungsten wire filament, tends to be rather large and electrons are emitted radially outward from all surfaces of the filament surface. The cup, therefore, must be designed to produce a very tailored electric potential distribution in the vacuum such that all electron trajectories are redirected from their initial divergent motion toward a very small focal spot on the anode surface. For design purposes it is usually sufficient to treat the coiled filament as a solid emitting cylinder, and to neglect detail at the level of individual turns of the coil. It is also usually sufficient to be concerned only with the focal spot width, rather than its complete two-dimensional shape because the focal spot length can be more-or-less independently set by emitter-cup changes which do not strongly alter the width. However, even with this design freedom, it is difficult in practice to design a cup which produces such tailored electric fields and leads to a small focal spot width. The present state-of-the-art is represented by filament coils of major diameter around 1 millimeter which can be focused onto a 0.1 millimeter-wide focal spot on the anode, i.e., a beam compression ratio of 10.
A universal limitation of electron emitters is that the net emission current as measured between the cathode and anode cannot be increased without bound simply by increasing the primary emission current of the emitter. As used herein, primary emission denotes electrons leaving the emitter surface and does not include any electrons which return to the surface. More precisely, the net emission current density at the emitter is limited. The net emission current is the primary emission current less any electron current returning to the emitter surface. At very low primary emission current density, corresponding to low heating current and low emitter temperature for a thermionic emitter, the net emission current density will increase in nearly direct proportion to any increase in primary emission current density. Conversely, at very high primary emission current density, the electron density immediately in front of the emitter surface is so high that the self-charge of the electron cloud completely counteracts the electric field at the emitter surface caused by the cathode-anode potential difference. This latter condition is referred to as a saturated emitter; further increases in primary current density do not appreciably increase the net emission current. Between these two extremes is a smooth transition where increases in primary emission current density lead to less than proportionate increases in net emission current, and practical x-ray tubes often operate in this transition regime. All electron emitters are limited by this fundamental process, independent of the emitter material and emission mechanism. A useful figure-of-merit for characterizing the overall capability of a cathode is its perveance, defined as the ratio I/V.sup.3/2, where I is the net electron current and V is the potential difference between the cathode and anode. Additionally, the self-charge of the electrons in the vacuum can alter the electric potential and can cause undesirable changes such as enlargement of the focal spot size, sometimes referred to as blooming. Thus, cathode designs which are capable of meeting design goals on net current and yet which operate far below their inherent saturation current density can be advantageous. Finally, there is ordinarily a tradeoff between the useful life of a thermionic emitter and its operating temperature such that it can be desirable to operate the emitter at a lower temperature, and hence a lower primary emission current density.
A further disadvantage of typical cathode designs is that the cup design needed to properly focus the electrons results in a significant reduction in the saturation current of the cathode, and hence the maximum obtainable x-ray emission over that which would be expected if the filament were operated in free space apart from the cup. In particular, the aforementioned requirement that the initial, radially directed electron distribution from a helical coil filament be redirected onto the small focal spot leads one to place the filament emitter into a rather narrow slot. Unfortunately, this reduces the electric field normal to the front surface of the filament significantly below the average electric field present in the cathode-anode gap, which is on the order of V/L. Here, V is the electric potential between the cathode and anode, and L is the cathode-anode spacing. The electric field strength normal to the emitter surface, in the absence of any electron emission, determines the saturation current density of each point on the filament surface. Further, the electric field strength normal to the emitter surface is highest only on that portion of the filament which is closest to the anode; it decreases away from this one point; hence, the saturation current density decreases away from this one particular location. In principle, the emitting area may always be increased to obtain a higher total emission current, but as noted hereinabove, it is difficult to increase the filament size without also undesirably increasing the focal spot size.
A further limitation of conventional filament-cup cathode designs is that it is quite difficult in practice to form anything resembling a laminar electron beam wherein the trajectories of electrons emitted from various locations on the filament do not cross each other as they move from the cathode to the anode As a result, the spatial distribution of current density across the width of the focal spot on the anode surface is not the gaussian distribution which would lead to the best modulation transfer function and hence the best image quality. Instead, the focal spot current distribution is typically double-peaked. The peak electron current density within the focal spot on the target is limited by the peak temperature capability of the anode. Therefore, to the extent that the actual peak current density exceeds that of an otherwise equivalent gaussian spatial distribution for a given anode design, the total current, and hence the maximum achievable x-ray fluence, will be reduced. It is not necessary that the electron flow be close to laminar in order to create the desirable gaussian spatial distribution of electron current, but the highly nonlaminar nature of the electron beam created by conventional filament-cup cathode designs makes the formation of a gaussian focal spot quite difficult in practice.
An emitter-cup cathode which simultaneously provides higher emission current, smaller focal spot width, and better modulation transfer function has been heretofore unavailable. This has been disadvantageous in, for example, medical x-ray tubes for mammographic imaging applications. Because of the desired low-energy x-ray spectrum for these applications, the electric field between the cathode and anode is relatively low (e.g., a 30 kilovolt potential difference applied over a cathode-anode gap of 7 millimeters), and the corresponding saturation current can be quite low, leading to low x-ray intensity and long exposure times. Further, mammographic imaging demands some of the smallest focal spot sizes among all medical applications in order to reliably detect microcalcifications indicative of the earliest stages of breast cancer.
Accordingly, it is desirable to provide an emitter-cup x-ray tube cathode which overcomes the hereinabove described disadvantages.